Low temperature method for producing amorphous boron-carbon deposits

ABSTRACT

PROCESS FOR PRODUCING AN AMORPHOUS BORON-CARBON DEPOSIT ON A FILAMENTARY SUBSTRATE AT A TEMPERATURE BELOW 1,000*C. BY HEATING THE FILAMENTARY SUBSTRATE IN A VACUUM CHAMBER TO A TEMPERATURE ABOVE 700*C., PREFERABLY IN THE RANGE 750-850*C., AND CONTACTING IT WITH A HYDROCARBON-SUBSTITUTED BORANE GAS SUCH AS ETHYL DECABORANE OR TRIETHY BORANE.

United States Patent LOW TEMPERATURE METHOD FOR PRODUCING AMORPHOUSBORON-CARBON DEPOSITS Joseph J. Gebhardt, Malvern, Pa., assignor toGeneral Electric Company, a corporation of New York No Drawing. FiledSept. 28, 1966, Ser. No. 582,483 Int. Cl. C23c 13/04 U.S. Cl. 117-46 5Claims ABSTRACT OF THE DISCLOSURE Process for producing an amorphousboron-carbon de posit on a filamentary substrate at a temperature below1,000" C. by heating the filamentary substrate in a vac uum chamber to atemperature above 700 C., preferably in the range 750-850 C., andcontacting it with a hydrocarbon-substituted borane gas such as ethyldecaborane or triethyl borane.

This invention relates to a process for producing amorphous boron-carbondeposits and more particularly to a process in which amorphousboron-carbon deposits are produced at relatively low temperatures.

Amorphous boron-carbon deposits, either in the boron carbide form or innon-stoichiometric mixtures of boron and carbon, have been found to havehighly desirable mechanical propertiers. In particular, these materialsare known for their extremely high tensile strength and modulus ofelasticity even at very high temperatures. It should be noted that whilethis material is referred to herein as amorphous, this term is notintended to signify the complete absence of a crystalline structure inthe deposit but instead indicates only that no crystalline structure isdiscernible by the presently available techniques, such as X-raydiffraction, etc.

Despite the highly desirable mechanical properties of amorphousboron-carbon deposits, utilization of these materials has been limitedgenerally by the fact that prior art processes for producing suchmaterials have required process temperatures which would destroy orcause degradation of many substrate materials. Further, these processeshave been relatively slow, and have not been capable of consistentlyproducing high quality deposits.

For example, the most common method for producing amorphous boron-carbondeposits heretofore has been to heat the substrate to temperatures above1,100 C. in the presence of boron trichloride, methane, and hydrogen.Substrates which undergo undesirable physical or chemical changes inthis temperature range cannot be used in this process.

It is an object of the present invention therefore to provide a methodfor producing amorphous boron-carbon deposits at a relatively lowtemperature.

Another object of this invention is to provide a practical lowtemperature process for producing amorphous boron-carbon deposits in aform which can be utilized as a structural material reinforcement.

These and other objects are met, in accordance with the presentinvention, by a process which comprises, briefly, heating a substratematerial to a temperature in the range from 700-900" C. at a very lowpressure and contacting the substrate material with ahydrocarbon-substituted borane gas. In the preferred form of the presentinvention a filamentary substrate is contacted with an alkyl-substitutedborane at a temperature of 750-850 C. and a pressure of from -25 torr.

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention,this invention may be better understood from the following description.

3,556,834 Patented Jan. 19, 1971 ICC With regard generally to thesubstituted boranes which constitute the source of both boron and carbonin the present invention, a variety of hydrocarbon-substituted boranesmaybe useful but the preferred materials are the lower alkyl-substitutedboranes, such as ethyl decaborane and triethyl borane. These materialsare relatively stable at ordinary temperatures and apparently undergocontrolled decomposition at the reaction temperature and pressure of thepresent process. Although the specific mode of decomposition is unknown,it is likely that numerous intermediate products are formed as thedecomposition proceeds.

The present invention generally requires a very low pressure in order toeffect controlled decomposition and to minimize extraneous sidereactions. More specifically, the pressure at which the process of thepresent invention is conducted should be below 25 torr. The pressurerange from 10 to 25 torr is preferred. It should be noted that a torr isequal to a pressure of 1 millimeter of mercury, absolute.

The objects of the present invention make it clear that this inventionis directed to a low temperature process. Generally, this is a processin which amorphous boroncarbon deposits are produced on substrates whichneed not be heated in excess of 1,000 C. Although substrate temperaturesthroughout the range from 700-900 C. have been found to be effective toproduce amorphorus boroncarbon deposits in accordance with the presentinvention, temperatures in the range from 0-85 0 C. are preferred.

The effectiveness of the present invention to produce deposits in thetemperature range below 1,000 C. is an important feature of the processtaught herein. It permits the production of amorphous boron-carbondeposits on many substrates which would undergo undesirable physical orchemical changes at temperatures below 1,000 C. The latter categoryincludes metallic substrates, which may undergo crystalline changes orexhibit a loss in ductility if they are heated above 1,000 C., and alsoincludes substrates such as silica which would deteriorate attemperatures above 1,000 C. The latter is a particularly desirablesubstrate for use in producing low density boroncarbon filaments. Suchfilaments, produced, in accordance with the present invention, by thedeposition of amorphous boron-carbon on a filamentary silica substrate,have excellent potential as the reinforcing constituent of compositestructural materials.

The quality of these products has been determined by visual observationas well as by microscopic analysis which showed the deposits to berelatively smooth and free of imperfections.

As an example of the present invention, an experiment was conducted inwhich a tungsten filament, 1 mil in diameter, was mounted in a tubularquartz reactor having an internal diameter of inches and a length of 2inches. Inlet and outlet tubes were connected to the reactor, mounted atright angles to the tubular axis of the reactor. The outlet tube wasconnected to a vacuum-producing apparatus and the inlet to a source oftriethyl borane. When the pressure had been reduced to about 18-22 torr,and electrical contacts in the tubular reactor had heated the tungstenfilament to about 900 C., triethyl borane was admitted to the reactor ata relatively high rate. For thirty minutes, the triethyl borane wascontinuously passed through the reactor while the pressure in thereactor Was held at 18-22 torr. During this time, the filament had grownto a diameter of 3.27 mils, roughly at a diameter growth rate of 4.5mils per hour. Later, this experiment was repeated at a lowertemperature, about 700-800 C., and at a slightly higher feed rate oftriethyl borane. The pressure in the latter experiment was held at 13-15torr. In this later experiment, a filament 2.64 mils in diameter 3 wasproduced in 13 minutes at a diameter growth rate of 8 mils per hour.

In still another example of the present invention, a low temperaturefilamentary substrate, specifically silica, .was used. The silicasubstrate was 0.64 mil in diameter and had a thin carbon coating on itssurface. Again, the substrate filament was resistively heated to 700800C., and the pressure was held at 13-15 torr. In this example, adeposition rate of about .7 mil per hour was attained. In a modificationof this example, another experiment was conducted in which ethyldecaborane was contacted with a silica substrate at 800-850 C. This alsoresulted in the deposition of amorphous boron-carbon on the silicafilament.

The filaments produced in all the above examples were typically hardand, under X-ray analysis, devoid of any discernible crystal structure.In order to ascertain that the deposits were not pure boron, an attemptwas made to etch these deposits with 50% hydrogen peroxide at 100 C. Noetching occurred, which indicated that the deposits were not simplyboron. Since carbon would not be expected to be deposited at the lowtemperatures of these experiments, it is also unlikely that the depositsproduced were simply carbon. Therefore, it was concluded that thedeposits produced in these experiments were amorphous boron-carbondeposits. A portion of these deposits may have been boron carbide butthis could not be proven.

Since high quality, low density amorphorus boroncarbon filaments arelikely to be highly desirable as the reinforcement constituent ofcomposite structural materials, an adaptation of the present process tosome form of continuous deposition apparatus will probably be requiredto provide such filaments in the quantities required. In one suchadaptation demonstrating the practicality of a continuous form of thepresent invention, the filamentary substrate was passed continuouslythrough the length of a tubular reactor in which it was heated and exposed to a continuously flowing hydrocarbon-substituted borane. Anamorphous boron-carbon filament was obtained.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A process for producing a high strength filament consistingessentially of an amorphous boron-carbon deposit, said processcomprising placing a filamentary substrate material in a vacuum chamber,evacuating said chamber to a pressure below 25 torr, heating saidsubstrate material to a temperature in the range 700-900 C., and contacting said substrate material with ethyl decaborane.

2. A process, such as that recited in claim 1, wherein said chamber isevacuated to a pressure in the range from 10 to 25 torr.

3. A process, such as that recited in claim 1, wherein said processoperates continuously on a substrate material of indefinite length whichpasses through said vacuum chamber after said chamber has been evacuatedand as said gas is also passed continuously through said vacuum chamber.

4. A process, such as that recited in claim 1, wherein said temperatureis in the range from 750850 C.

5. A process, such as that recited in claim 1, wherein said substratematerial is filamentary silica.

References Cited UNITED STATES PATENTS 2,484,519 10/1949 Martin 117-1062,671,735 3/1954 Grisdale et al 11746 2,989,421 6/1961 Novak l17123A3,142,707 7/1964 Obenland 260606.5 3,347,931 10/1967 Wunz, In, et a1.260606.5 3,398,013 8/1968 Krochmal et a1 117-106 OTHER REFERENCES Powellet al., Vapor Plating, 1955, pp. 9-12 and 15 relied upon.

Coates, Organo-Metallic Compounds, 1960, pp. 94 to 98 relied upon.

ALFRED L. LEAVITT, Primary Examiner I. R. BATI EN, 111., AssistantExaminer U.S. Cl. X.R.

